The subject matter of the present invention relates to shock absorbers adapted for use in a wellbore, and more particularly, to a shock absorber adapted to be connected between a production valve and a perforating gun of a tool string disposed in a wellbore for absorbing shock resulting from detonation of the perforating gun, the shock absorber including a frangible breakup element which shatters in response to a detonation wave passing therethrough, the shock absorber absorbing the shock from the detonation of the perforating gun only when the frangible breakup element shatters.
While many types of shock absorbers have been used downhole in a wellbore, all have similar problems. Most shock absorbers use dual elements, an upward element for absorbing an upward shock and a downward element for absorbing a downward shock. The downward element is often preloaded by the weight of the tool string below the element, and the upward element is often preloaded when the tool string encounters a bridge or tight spot when being lowered into the borehole. Some shock absorbers, such as U.S. Pat. Nos. 4,693,317 and 4,817,710 to Edwards et al, attempted to reduce or eliminate this preloading effect by preloading both the upward and downward elements; this would assist in preventing a shock from having a running start or rebounding at the absorbing elements. This shock absorber was not sealed thereby limiting its use to certain wells where it could be run above the gun string and below a packer. Preloading of the elements also caused the shock absorber to vary in length when made up in a string, due to the different weights of equipment run below the shock absorber on the tool string. This variation in length made it difficult to determine the precise location of the perforating guns in the wellbore. Furthermore, current shock absorbers cannot be run within an explosive train of the tool string since most are not sealed. In addition, most current shock absorbers use two shock elements which are usable for a single job, making it expensive to repair and maintain. However, one shock absorber, which is not subject to aforementioned preloading effects when being raised or lowered in a borehole, is disclosed in U.S. Pat. No. 5,131,470 to Miszewski et al. The Miszewski shock absorber has two energy absorbing elements, one for upward shock and one for downward shock, and the shock absorber can be run within an explosive train of the tool string. The shock absorber is not subject to preloading due primarily to a physical connection which exists between an inner and outer housing, the two shock absorbing elements being disposed between the two housings. The physical connection is broken in response to a detonation wave in a detonating cord, the detonation wave detonating a donut shaped breakup charge, a jet from the breakup charge breaking the physical connection. Such breakup charges are not very reliable. Thus, another method and apparatus is needed to avoid the aforementioned preloading effects which exist when the shock absorber is raised and lowered in a wellbore, yet to selectively allow the shock absorber to absorb a shock which results when a perforating gun is detonated in a wellbore.